base-files: sysfixtime: Fix time on the fake RTC

base-files: sysfixtime: Fix time on the fake RTC

On some devices the chip has RTC but no battery save time.
This leads back to getting the wrong time
and skipping the check of the last file modification date.

This commit ensures that the file time is checked even
if the RTC exists.
which would ordinarily return an approbiate
system time used for e.g. certificate generation.

Tested-on: NanoPi R2S

Signed-off-by: Yuan Tao <ty@wevs.org>